1. Field of the Invention
The present invention relates to a method permitting the rapid and highly precise measurement of the minority carrier diffusion length in a silicon wafer by stabilizing the silicon wafer surface shortly following surface treatment in the course of measuring the minority carrier diffusion length by the surface photovoltage method; and to a method of manufacturing a silicon wafer using this method.
2. Discussion of the Background
Typically, when crystal defects are present in silicon wafers (also referred to hereinafter as “wafers”) or wafers are contaminated with metal impurities, the device characteristics of the finished product are negatively affected. Thus, the measurement of electrical properties such as the recombination lifetime and the diffusion length of the minority carrier within the wafer has been proposed, to facilitate the evaluation of such defects and contaminants.
In the evaluation of electrical properties, the evaluation of diffusion length by the surface photovoltage (SPV) method is widely employed to assess high levels of contamination by metals deep within the wafer, such as iron (see Japanese Unexamined Patent Publication (KOKAI) Heisei No. 6-69301, which is expressly incorporated herein by reference in its entirety). The SPV method is a good measurement method in that it allows rapid, non-contact measurement without damaging the wafer.
A summary of the method of measuring the minority carrier diffusion length in a silicon wafer by the SPV method is given below.
First, the silicon wafer is surface-treated (surface charge treated). In the case of a p-type wafer, the surface treatment is conducted by immersing the silicon wafer in hydrofluoric acid (HF) for a prescribed period of time. This surface treatment produces band bending near the surface. In this state, minority carriers excited by irradiation through the silicon surface by light having an energy level greater than the band gap of the silicon can be collected at the surface by the electric field caused by band bending, producing a surface photovoltage (SPV). When the number of carriers injected by the light is not so large (generally 1E10 to 1E13/cm3), a surface photovoltage is generated in proportion to the excess minority carrier density at the surface. When the wavelength of the light being irradiated is changed, the density of the minority carrier being generated, that is, the SPV value, changes with the penetration depth of the light (coefficient of light absorption α (alpha)). A quantity of light is selected at which the relation between the quantity of light (φ(phi)F), SPV, and coefficient of light absorption α at various wavelengths is φF/SPVc∝1/α. Once this relation has been plotted, the minority carrier diffusion length can be obtained as the point of intersection with the 1/α axis at φF/SPV=0.
When the wafer contains metal impurities, the minority carrier diffusion length typically decreases. However, when a metal impurity such as iron (Fe) is contained, for example, in a boron-doped p-type silicon wafer, electrostatic forces cause Fe to pair off with boron in Fe—B pairs. As a result, there is almost no effect on the minority carrier diffusion length. By contrast, when the silicon wafer surface is irradiated with light to dissociate pairs (such as Fe—B pairs) forming between dopants and contaminant metals, Fe does affect the minority carrier diffusion length and the minority carrier diffusion length decreases. The presence and concentration of contaminant metals can be determined based on such differences in minority carrier diffusion length before and after dissociation treatment.
However, the wafer surface is not stable immediately following the above surface treatment. Thus, it is impossible to obtain accurate measurements when the diffusion length is measured immediately following surface treatment. For this reason, a wafer will normally be left standing for a prescribed period of time after surface treatment, with measurement being conducted once the wafer surface has stabilized. However, several hours are normally required for the wafer surface to stabilize, precluding rapid measurement. Further, it is difficult to determine how long a wafer must be left standing before the wafer surface will stabilize. There is also a problem in that highly reliable data cannot be obtained when measurement is conducted with the wafer surface in a state of inadequate stabilization.